1. Field of the Invention
The present invention relates to thermal printing, and more particularly, to a printing method and apparatus utilizing feedback control to compensate for variations in the transport rate of the print media.
2. Description of Related Art
In the field of bar code symbology, parallel bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. To read the bar code, the bars and spaces are scanned by a light source, such as a laser. Since the bars and spaces have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the laser light after it has reflected from the bar code.
Bar codes are often printed onto paper substrate labels that can be affixed to the objects intended to be identified. The paper substrate labels typically comprise a face material onto which the bar code is printed with an adhesive backing layer applied to an opposite surface of the face material that permits the labels to be affixed to an object. The face material may be further laminated onto a release liner having a low-stick surface that allows the label to be removed easily. After the label is printed, the user can simply peel off the face material from the release liner, and apply the label onto an object. In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking or blurring of the bar code. Moreover, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process, otherwise the labels will not stick properly to the object.
In view of these demanding printing requirements, bar codes are often printed using thermal printing techniques. In thermal printing, the face material of the paper substrate labels is impregnated with a thermally sensitive chemical that is reactive upon exposure to heat for a period of time. Alternatively, an ink ribbon may be utilized that is selectively heated to transfer ink to the face material. The labels are drawn across a thermal print head having linearly disposed printing elements that extend across a width of the labels. The printing elements are selectively activated in accordance with instructions from a controller to heat localized areas of the substrate or ink ribbon, thereby creating a dark image by a chemical reaction brought on by the heat. As the labels are drawn through a print region between a platen and the thermal print head, the bar code is printed onto the face material. Other images, such as text or graphics characters, can also be printed in the same manner.
The thermal printer includes a mechanism for transporting the labels from a supply spool to the print region. The transporting mechanism controls the feed rate of the labels from the spool, and maintains a positive tension on the labels so as to prevent their wrinkling which could cause a defect in the printed bar code. The transporting rate must be controlled so that it synchronizes with the activation rate of the printing elements in order to print the labels accurately. If the transport rate of the labels were to momentarily slow down, stop or speed up while the printing elements were activated, the printing would be disrupted and, in the worst case, the substrate material of the labels could be burned or torn. Thus, the paper substrate labels are transported at a substantially uniform rate in order to obtain substantially defect-free printing.
In a new formulation of the paper substrate labels, the release liner is eliminated, and the labels are simply wound onto themselves with the backing layer adhering directly to the face material of subsequent labels. These so-called "linerless" labels include an adhesive backing layer specifically formulated to prevent formation of a permanent adhesive bond, enabling the labels to be subsequently peeled off without damaging the face material. Linerless labels are more convenient than conventional labels for certain types of applications, and elimination of the release liner reduces a substantial amount of waste material normally generated in the labeling process and increases the supply available for printing.
Nevertheless, the adhesive backing layer exerts a force opposite in direction from the transport force applied by the transporting mechanism, referred to as the "payout" force. This payout force must be counteracted by the transport mechanism in order to draw the labels from the spool to the print region. For example, the magnitude of the payout force typically decreases with the decreasing diameter of the label spool as the label supply is exhausted during the printing process. In practice, the variable payout force is often difficult to predict, and cannot be adequately compensated for by the transport mechanism. As a result, a uniform transport rate cannot be achieved and the label print quality becomes degraded.
In a similar manner, the conventional paper substrate labels using release liners (i.e., "linered" labels) are also susceptible to variable payout force. The release liner can occasionally slip off a region of the label causing the label to stick to the label disposed one layer below the current label within the spool or adhesive bleed can cause the liners to stick together. The uneven adhesion force causes variations in the payout of the paper substrate labels.
The problems associated with variations in payout force become more pronounced as print speeds become higher, label spools become longer and more specialized print media is used. In addition, many modern applications are requiring higher print qualities that are sensitive to even smaller variations in payout force. Further, modern printers are allowing for more sophisticated label movements, such as retract, and are including accessories such as cutters, self-strip mechanisms, and batch take-ups. These new movements and accessories, as well as the wear on individual pieces of the transport mechanism, can cause additional variations on the transport rate of the labels through the transporting mechanism.
Accordingly, it would be desirable to provide a transporting mechanism for a thermal printer that is capable of taking advantage of either linered labels or the new formulation of linerless paper substrate labels by compensating for the variable payout force applied by the labels. It would also be desirable to provide a method for compensating for variations in the transport rate of the print media that are caused by variables such as the print media type, individual parts of the transporting mechanism and print modes. Additionally, it would be desirable if this method for compensating for transport rate variations could detect and compensate for unforeseen variables that cause variations in payout force. Ideally, the transporting mechanism would be capable of providing a uniform payout rate of the reformulated labels and be compatible with conventional linered labels so as to synchronize with the activation rate of the printing elements during print operations.